Ocean

Photo by: Ovidiu Iordachi

Oceans are large bodies of salt water that surround Earth's
continents and occupy the basins between them. The four major oceans of
the world are the Atlantic, Arctic, Indian, and Pacific. These
interconnected oceans are further divided into smaller regions of water
called seas, gulfs, and bays.

The combined oceans cover almost 71 percent of Earth's surface, or
about 139,400,000 square miles (361,000,000 square kilometers). The
average temperature of the world's oceans is 39°F
(3.9°C). The average depth is 12,230 feet (3,730 meters).

Waves erode the land upon which they land as well as the ocean floor.
(Reproduced by permission of

The Stock Market

.)

Origin of ocean water

One scientific theory about the origin of ocean water states that as Earth
formed from a cloud of gas and dust more than 4.5 billion years ago, a
huge amount of lighter elements (including hydrogen and oxygen) became
trapped inside the molten interior of the young planet. During the first
one to two billion years after Earth's formation, these elemental
gases rose through thousands of miles of molten and melting rock to erupt
on the surface through volcanoes and fissures (long narrow cracks).

Within the planet and above the surface, oxygen combined with hydrogen to
form water. Enormous quantities of water shrouded the globe as an
incredibly dense atmosphere of water vapor. Near the top of the
atmosphere, where heat could be lost to outer space, water vapor condensed
to liquid and fell back into the water vapor layer below, cooling the
layer. This atmospheric cooling process continued until the first
raindrops fell to the young Earth's surface and flashed into steam.
This was the beginning of a fantastic rainstorm that, with the passage of
time, gradually filled the ocean basins.

Words to Know

Fracture zone:
Faults in the ocean floor that form at nearly right angles to the
ocean's major ridges.

Guyot:
An extinct, submarine volcano with a flat top.

Ridge:
Very long underwater mountain ranges created as a by-product of
seafloor spreading.

Rift:
Crevice that runs down the middle of a ridge.

Seafloor spreading:
Process whereby new oceanic crust is created at ridges.

Seamount:
Active or inactive submarine volcano.

Cosmic rain.
In mid-1997, however, scientists offered a new theory on the how the
oceans possibly filled in. The National Aeronautics and Space
Administration's Polar satellite, launched in early 1996,
discovered
that small comets about 40 feet (12 meters) in diameter are bombarding
Earth's atmosphere at a rate of about 43,000 a day. These comets
break up into icy fragments at heights 600 to 15,000 miles (960 to 24,000
kilometers) above ground. Sunlight then vaporizes these fragments into
huge clouds, which condense into rain as they sink lower in the
atmosphere.

Scientists calculate that this cosmic rain adds one inch of water to
Earth's surface every 10,000 to 20,000 years. This amount of water
could have been enough to fill the oceans if these comets have been
entering Earth's atmosphere since the planet's beginning 4.5
billion years ago.

Ocean basin

Ocean basins are that part of Earth's surface that extends seaward
from the continental margins (underwater plains connected to continents,
separating them from the deep ocean floor). Basins range from an average
water depth of about 6,500 feet (2,000 meters) down into the deepest
trenches. Ocean basins cover about 70 percent of the total ocean area.

The familiar landscapes of continents are mirrored, and generally
magnified, by similar features in the ocean basin. The largest underwater
mountains, for example, are higher than those on the continents.
Underwater plains are flatter and more extensive than those on the
continents. All basins contain certain common features that include
oceanic ridges, trenches, fracture zones, abyssal plains, and volcanic
cones.

Oceanic ridges.
Enormous mountain ranges, or oceanic ridges, cover the ocean floor. The
Mid-Atlantic Ridge, for example, begins at the tip of Greenland, runs down
the center of the Atlantic Ocean between the Americas on the west and
Africa on the east, and ends at the southern tip of the African continent.
At that point, it stretches around the eastern edge of Africa, where it
becomes the Mid-Indian Ridge. That ridge continues eastward, making
connections with other ridges that eventually end along the western
coastline of South and Central America. Some scientists say this is a
single oceanic ridge that encircles Earth, one that stretches a total of
more than 40,000 miles (65,000 kilometers).

In most locations, oceanic ridges are 6,500 feet (2,000 meters) or more
below the surface of the oceans. In a few places, however, they actually
extend above sea level and form islands. Iceland (in the North Atlantic),
the Azores (about 900 miles [about 1,500 kilometers] off the coast of
Portugal), and Tristan de Cunha (in the South Atlantic midway between
southern Africa and South America) are examples of such islands.

Running along the middle of an oceanic ridge, there is often a deep
crevice known as a rift, or median valley. This central rift can plunge as
far as 6,500 feet (2,000 meters) below the top of the ridge that surrounds
it. Scientists believe ocean ridges are formed when molten rock, or magma,
escapes from Earth's interior to form the seafloor, a process known
as seafloor spreading. Rifts may be the specific parts of the ridges where
the magma escapes.

Trenches.
Trenches are long, narrow, canyonlike structures, most often found next
to a continental margin. They occur much more commonly in the Pacific than
in any of the other oceans. The deepest trench on Earth is the Mariana
Trench, which runs from the coast of Japan south and then west toward the
Philippine Islands—a distance of about 1,580 miles (2,540
kilometers). Its deepest spot is 36,198 feet (11,033 meters) below sea
level. The longest trench is located along the coast of Peru and Chile.
Its total length is 3,700 miles (5,950 kilometers) and it has a maximum
depth of 26,420 feet (8,050 meters). Earthquakes and volcanic activity are
commonly associated with trenches.

Fracture zones.
Fracture zones are regions where sections of the ocean floor slide past
each other, relieving tension created by seafloor spreading at the ocean
ridges. Ocean crust in a fracture zone looks like it has

Underwater ridge of the Juan de Fuca plate off the coast of
Washington State. These chimneylike structures on the ocean floor are
shaped by emissions of sulfides from deep beneath Earth's
crust.
(Reproduced by permission of

U.S. Geological Survey Photographic Library

.)

been sliced up by a giant knife. The faults in a zone usually cut across
ocean ridges, often nearly at right angles to the ridge. A map of the
North Atlantic Ocean basin, for example, shows the Mid-Atlantic Ridge
traveling from north to south across the middle of the basin, with dozens
of fracture zones cutting across the ridge from east to west.

Abyssal plains.
Abyssal plains are relatively flat areas of the ocean basin with slopes
of less than one foot of elevation difference for each thousand feet of
distance. They tend to be found at depths of 13,000 to 16,000 feet (4,000
to 5,000 meters). Oceanographers believe that abyssal plains are so flat
because they are covered with sediments (clay, sand, and gravel) that have
been washed off the surface of the continents for hundreds of thousands of
years. On the abyssal plains, these layers of sediment have now covered up
any irregularities that may exist in the rock of the ocean floor beneath
them.

Abyssal plains found in the Atlantic and Indian Oceans tend to be more
extensive than those in the Pacific Ocean. One reason for this phenomenon
is that the majority of the world's largest rivers empty into
either the Atlantic or the Indian Oceans, providing both ocean basins with
an endless supply of the sediments from which abyssal plains are made.

Volcanic cones.
Ocean basins are alive with volcanic activity. Magma flows upward from
the mantle to the ocean bottom not only through rifts, but also through
numerous volcanoes and other openings in the ocean floor. Seamounts are
submarine volcanoes and can be either active or extinct. Guyots are
extinct volcanoes that were once above sea level but have since receded
below the surface. As they receded, wave or current action eroded the top
of the volcano to a flat surface.

Seamounts and guyots typically rise about 0.6 mile (1 kilometer) above the
ocean floor. One of the largest known seamounts is Great Meteor Seamount
in the northeastern part of the Atlantic Ocean. It extends to a height of
more than 1,300 feet (4,000 meters) above the ocean floor.